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#3 Exhaust not hot

  • Thread starter Thread starter dbarnes42
  • Start date Start date
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dbarnes42

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Great website! This is my first time in an internet forum so I will try and explain this the best I can. I have a 79 GS850 that was rescued from a barn. It is completly stock, with 30K miles. Since I have had it, I have cleaned (dipped), rebuilt (new O rings) and synched the carbs, all using info off of this website. I have checked the timing, adjusted the points, installed new intake boots and o rings, adjusted the valves, changed all of the fluids and cleaned the gas tank. I had installed a in-line fuel filter. Last weekend I was riding and it was running great. I stopped to fill up, putting the bike on the side stand. Shortly after (maybe 5 miles), the bike started to run bad! It was spitting and sputtering and with full throttle had no power. I put it back in the garage and took the tank off and pulled the petcock off. The petcock was really dirty and there was even a piece of paper towel in the tank. The tank was cleaned again, this time using POR-15. My problem is now the #3 exhaust pipe is not getting hot. I pulled the plug and checked for spark, which is good since I have done the coil relay mod. I put the carbs back on and removed the other spark plug wires and the bike would not start. Is it possible that I need to remove the carbs and go through them again or is there something else I need to look for?
 
i would think you either have no fuel going into that cylinder. did you see if you had spark on that cylinder?
 
Does the vacuum line go to #3? If so pull it off at the carb and see if fuel is in the vacuum line. Sounds like you may have a bad diaphram in the petcock. If not maybe a bad plug cap. If fuel has been leaking through the vacuum port you may have fuel in your crankcase. If that's the case an oil change is in order.
 
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Hi and welcome.
If you're getting a spark on No3 chances are you're not getting fuel through the carb. I'd advise pulling them and cleaning again.

Worth checking beforehand that you haven't accidentally pulled the vacuum pipe of the carb though. Also worth checking your tap diaphragm is working ok and that you're not sucking fuel down the vacuum line.
 
Thanks for all of the great information so far. Yes, the vacum line does go to the #3 carb. I was planning on changing the oil anyway because I had rean on this forum about gas getting into the crankcase. The #3 spark plug did have a spark on it, with the plug resting on the engine. I know that the petcock is vacum operated so os there a way to check and see if it is workin properly? I also know that I can leave it in "PRI" but this is not a good idea in case I forget to turn it off.
 
dbarnes42 said:
Thanks for all of the great information so far. Yes, the vacum line does go to the #3 carb. I was planning on changing the oil anyway because I had rean on this forum about gas getting into the crankcase. The #3 spark plug did have a spark on it, with the plug resting on the engine. I know that the petcock is vacum operated so os there a way to check and see if it is workin properly? I also know that I can leave it in "PRI" but this is not a good idea in case I forget to turn it off.
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Don't leave the petcock on prime. If you have a float not seating properly you will have a puddle of gas under your bike and if a source of flame is anywhere near everything goes up in smoke. If the diaphram is bad there will be fuel in the vacuum line. Pull it off at the carb to see if fuel comes out.
 
I will not leave it on prime. Once again thanks for all of the great information.
 
A gas-fouled plug may not fire under combustion pressure. I would first swap plugs 3 & 4 to see if it is the plug (#3 getting warm?). To test the diaphram, pinch the vacuum hose (needle-nose pliers, zip-tied handle) and run it on prime up and down the street. Then check the plugs. If #3 is clean, then it probably is the diaphram.
 
If your tank was dirty you need to reclean the ports that lead to the inlet valve. (float seat) I bet you have some gunk in there.
Also the petcock vacuum line does go to the #3. Check the vacuum line for gas.
 
OK, I am really at a loss now. I took the carbs off and celaned them again. Got everything back together and the #3 pipe is still not getting warm. Just out of curiosity, I swapped the #2 and #3 plug wires. The pipe still did not get warm. When I had the carbs off and was cleaning them, I noticed that on carbs #1, 3, and 4 I could see light coming through the small hole where the pilot fuel screw goes. Is this normal? Also, I had done the coil relay mod but I think I may have messed something up becasue I am still getting a voltage drop out of the coils. I followed one of the set of directions off of this webstite, except I have the following setup - one orange/white went to the back of the bike and another ornage/white went to the coils, where it was soldered together to make two wires going to the coils. Anyone have any ides on either of these problems.
 
If your carbs are way out of sync it's possible that the conditions you describe can occur. I know, it's happened to me. Did you bench sync them?
 
Those holes for the pilot screws should have light coming out. It is very common for the tip to break off in there. BUT that will affect cylinder 2 (according to what you said). You need to get in there and find out what's up: do all your pilot screws have pointy tips?

The coil mod will use those orange/white wires as the trigger to pass through full voltage directly from the battery. Is that how you are wired?

Now, for your cold cylinder #3, you need to take concrete steps to narrow down your search. Don't lock in on the carbs.

Swapping plugs was an easy first step: it sounds like your plug was not just gas fouled.

You can swap #2 and #3 plug wires, they fire at the same time. This will eliminate a plug lead problem.

Did you pinch the vacuum line and test ride? That would eliminate the diaphram problem.

Do you have compression in that cylinder?

Did you visually inspect the synch when the carbs were off?

Did you spray WD40 around the boot to check for air leaks?

I would check these things first, then move to the carbs

With the bike on the centerstand after running, drain and measure the gas from bowls 3 and 4 (I cut the bottoms of oil bottles to use for this). Do they have the same amounts? If they do, then the gas delivery is probably ok.

Move the choke lever up and down. Make sure all plungers are moving equally.

If no problems are found up to this point, then it is time to go back into the carbs. You may be looking for fine differences: plugged pilot jets, drilled out pilot jets (crazy but true), drilled out main jets, bent needles, corroded emulsion tubes, plugged passages. Good luck.
 
Thanks for all of the info. I took the carbs back off last night and let them soak again. I blew them off with air and reassembled them. I did do a bench synch on them. I have not put them back on yet to see if the problem is fixed. As far as the plugs go, I swapped the wires for #2 and #3 and the pipe was still cold. This morning I swapped the actual plugs so I will check that out later. As for the coil mod, should both of the ornage/white wires that run to the kill switch be used as the trigger or just one of them? I will try every thing else listed to see if the pipe gets hot and keep everyone posted!
 
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